Sputtering protection for tantalum cathodes in plasma devices



Jung-2 24, 1969 R, W|NGERS0N 3,452,237

SPUTTERING PROTECTION FOR TANTALUM CATHODES IN PLASMA DEVICES Filed March 13, 1967 M28 F i5'- .E

INVENTOR. ,ea WIA/66e om *ETE-3 wf 'United States Patent O1 3,452,237 SPUTTERING PROTECTION FOR TANTALUM 4CATHODES IN PLASMA DEVICES Richard C. Wingerson, 1754A Arlin Place, Fairborn, Ohio 45324 Filed Mar. 13, 1967, Ser. No. 623,197 Int. Cl. H01j 17/06, 61/08 U.S. Cl. 313-211 3 Claims ABSTRACT OF THE DISCLOSURE Background of the invention The invention relates to the art of gaseous plasmas. In particular, it relates to the construction of the cathode electrode as used in plasma producing devices.

Such modern devices as arc heaters for hypersonic wind tunnels, magnetohydrodynamic electric power generators, electric arc thrusters for space, and testing devices for investigating the resistance of various materials to plasmas employ high intensity electric arcs in gases. The arcs in these devices are formed between anode and cathode electrodes in the presence of an ionzible gas. The cathode should be made of a metal having `a high melting point, good mechanical strength, particularly freedom from brittleness, and chemical inertness to the gas employed. Tantalum and argon satisfy these requirements to a great degree and have been widely employed in plasma devices.

A serious problem with cathodes, including tantalum cathodes, has been the relatively rapid erosion and consequent decrease in operating life of this electrode through bombardment by ions, a process called sputtering. Sputtering is a threshold effect, i.e. the ions must strike the cathode with a kinetic energy level above the threshold of the particular material of which the cathode is made. With this energy level they are able to eject particles from the crystal lattice of the cathode material and thereby erode the cathode. For a hot tantalum cathode in argon, the voltage required to sustain the arc is high enough that the positive ions accelerated thereby toward the cathode attain a kinetic energy level tabove the threshold value. In addition to erosion, the sputtering process results in brittleness and eventual breakage of the cathode.

Summary of the invention In accordance with the invention, damage to a tantalum cathode `as a result of sputtering is largely prevented and the useful life of the cathode greatly extended by forming a very thin, probably monatomic, film of gallium over the cathode surface. The effect of this ilm is to raise the sputtering threshold of tantalum in an argon arc above the energy level of the bombarding ions. Further, in accordance with the invention, this coating is accomplished simply by providing a small reservoir of gallium in contact with the tantalum cathode at a point where the temperature is below the boiling point of gallium.

Brief description of the drawing FIG. 1 shows the general arrangement of a plasma producing device using fa hollow cathode,

FIG. 2 shows in detail the cathode of FIG. 1 and the manner in which the invention may be applied to this cathode, and

3,452,237 Patented June 24, 1969 ice FIG. 3 shows the application of the invention to a filamentary cathode.

Description of the preferred embodiments FIG. 1 is an example of a plasma device to which the invention may be applied. The apparatus shown is for the purpose of testing the ability of a given material to withstand the effects of a high temperature plasma. The device comprises a sealed chamber 1 which communicates through a port 2 with vacuum apparatus 3 for maintaining a desired low pressure in the chamber. For argon, this pressure may be of the order of 10*5 mm. Hg.

-The arc forms between cathode 4 and anode 5. The cathode is a hollow tantalum tube supported at the end of a cathode assembly 6 and communicating with a tube 7. Argon gas from source 8 is introduced into the chamber 1 through tube 7 and the tubular cathode 4. The anode 5 may be in the form of a U-shaped copper tube, seen edgewise 'in FIG. 1, supported by a plate or flange 9 and introduced into the chamber through a port 10. A suitable liquid such as water or oil is circulated through the tube for cooling the anode.

The cathode assembly is held by a cathode support 11 of insulating material to permit a voltage to be established between the anode and cathode for sustaining the arc. A solenoid 12, of insulated copper tubing through Vwhich a coolant is circulated, surrounds the cathode end of the cathode assembly and establishes a magnetic field about the cathode 4.

The arc may Ibe initiated in any suitable manner such as by the temporary introduction of a high voltage tickler wire through tube 7 and cathode 4 into the space between the anode and cathode. Following the establishment of the arc, the tantalum cathode becomes incandescent and the argon atoms, which are being bled into the system through the tube 7 and the hollow cathode, are partially ionized within the cathode by collision with thermionic electrons emitted from the inner surface of the tubular cathode. These ions together with unionized atoms of argon emerge from the end of the cathode and are partially retained within the magnet by the boron nitride end cap 13. Under the' 'influence of the magnetic eld produced by solenoid 12 and the electric eld existing between the anode and the cathode, the charged particles within this space are caused to spiral and produce further ionization vby collision. This etfect increases the ionization eliiciency of the discharge and produces a denser plasma. The pressure differential between the interior of the magnet and the chamber 1 forces the plasma through the hole 14 in the boron nitride cap. Thermal motion of the particles then causes the plasma to difruse through and around the anode 5 into the region of the target 15.

The target electrode comprises two stainless steel cylinders each attached to a U-shaped copper tube support, only one cylinder and support 16 and 17, being visible in FIG. 1. Coolant circulates through the tubes for controlling the target temperature. The structure is protected from the plasma by a Pyrex glass tube 18 and a boron nitride disc 19 having holes to permit passage of the cylinders, The sample to be tested is attached to the cylinders in any suitable manner such as by screws. Electrical connection is made to the target electrode through the copper tubes. The electrode is normally held at a negartive potential relative to the anode to produce bombardment by the plasma ions.

FIG. 2 shows the details of the cathode end of the cathode assembly 6; This assembly comprises a cathode support 20 comprising three concentric brass tubes attached to a copper end block 23. The end block has an opening communicating with tube 7 and tapped to receive a compression tting 24 which supports the tubular cathode 4. The cathode support is protected from the 3 plasma by a Pyrex glass cover 25 and a boron nitride end cap'26. Tube 7 passes through the outer' end of the cathode assembly, as shown in FIG. 1, to permit the introduction of argon through this tube and the tubular cathode 4. Coolant is circulated through the cathode support in the spaces dened by tubes 21 and 22.

The cathode 4 is a tantalum tube. As stated earlier, the electric field existing between the anode and cathode causes positive plasma ions to be accelerated toward the cathode and to strike the cathode with kinetic energies exceeding the sputtering threshold of tantalum which is less than 100 ev. This results in rapid erosion of the cathode and an increase in its brittleness, leading eventually to destruction through extreme erosion or breakage.

In accordance with the invention, this erosion and brittleness is prevented or, at least, very greatly reduced by providing a thin ilm of gallium over the cathode surface. To this end, the cathode 4 in FIG. 2 is provided with an annular groove 27 in which is placed a small quantity of gallium, a liquid at room ltemperature, to provide a reservoir of this metal in Contact with the cathode. Due to cooling of the cathode support, this end of the cathode remains well Vbelow the boiling point of gallium, about 2000 C., so that the gallium is not lost through evaporation. vIt has been found that under these conditions gallium, due to its great wetting ability, will flow from the reservoir in a thin film over the inner surface of the tubular carthode, around the outer end and then over the outer surface until the exposed parts of the cathode are completely covered. This film, which at most is only a few atom diameters thick and is probably monatomic, adheres to and will not be lost from even the vhottest part of the cathode although the temperature may exceed the boiling point of gallium. This degree of adherence is due apparently to the extraordinarily strong bonds between the tantalum and gallium atoms at the surface of the cathode.

In a comparison test, an unprotected cathode of the type shown in FIG. 2 was badly eroded after about three hours of use, while a cathode operated under the same conditions but protected by a gallium film as described above showed no visible signs of damage in the same period. The exact mechanism by `which this protection is afforded is not fully understood at the present time, but it `appears to be related to the strong bonding 'between the tantalum and gallium atoms.

FIG. 3 illustrates the application of the invention to filamentary thermionic cathodes of the directly heated type which are also frequency used in plasma generators. Here a tantalum filament 28 is attached to cooled cathode supports 29 and 30 in one of which suitable provision is '-rnade for a reservoir of gallium in contact with the lament. The principle of operation is the same as for thelcathode in FIG. 2.

I claim:

1.l A plasma producing device having a tantalum cathode', means providing a small reservoir of molten gallium in contact with ai small area of the surface of the cathodo, and means for keeping the temperature of the surroundings of said reservoir of gallium sufficiently low to prevent any loss of gallium from the reservoir through evaporation.

2. Apparatus as claimed'in claim 1 in which the cathode is in the form of a tantalum tube supported at one endby a cooled cathode support and in which the gallium reservoir is located within the tubular cathode at its supported end.

3f. Apparatus as claimed in claim 1 in which the cathode is in the form of a tantalum filament supported at its ends by two cooled conductive cathode supports which also serve as electrical connections to the cathode for the passage therethrough of an electric current for heating the cathode to thermionic emission temperature, and in which said gallium reservoir is situated in one of said supports around its point of contact with the tantalum ilament.

References Cited UNITED STATES PATENTS 1,929,122 10/1933 Smith 313-212 X 2,498,841 2/1950 King 313-212 X 2,508,114 5/1950 ienne 313-211 X 3,354,644 11/1967 Moore 60-202 JAMES W. LAWRENCE, Primary Examiner.

R. F. HOSSFELD, Assistant Examiner.

U.S. Cl. X.R. 

